This invention relates to a window based stall avoidance mechanism in a HARQ of a wireless communication system. More particularly, this invention provides a dynamically calculated window size and a set of well-defined rules to improve system performance and resolve problems of the prior art.
The High Speed Downlink Packet Access (HSDPA) uses HARQ (Hybrid ARQ) and other techniques to handle high throughput. Meanwhile, by using Transmission Sequence Number (TSN) to track data blocks and by temporarily storing received data blocks based on their TSN and their priority classes in different reordering buffers, the receiver has the capacity for an in-sequence delivery of its receiving data blocks to high layer. However, stall situations happen when data blocks with higher TSN are kept in a particular reordering buffer and can not be delivered to upper layer because one or more data blocks of lower TSN in the same reordering buffer are missing. Several reasons may cause missing data blocks in a reordering buffer. First, some data blocks require more re-transmissions than others do before the receiver correctly decodes them. Second, transmission of a data block may be interrupted by the request to transmit data blocks with higher priority class. The interrupted data block may be re-initiated later when there is no more requests to transmit higher priority data blocks. Third, the transmitter may erroneously receive an ACK instead of a NACK returned by the receiver for a sent data block. Then the transmitter discards the particular data block from its retransmission buffer (i.e. the Note B) that causes the TSN of the data block become a “permanent” missing gap at the receiver. Last, after a predefined maximum number of retransmission of a data block, the transmitter may discard the data block from the retransmission buffer that will cause a “permanent” missing gap at the receiver.
Furthermore, because the number of bits allocated to represent a TSN is finite. For example, in a 6-bit TSN system, the TSN has the value between 0-63. In a high speed downlink HARQ setup, the transmitter may transfer hundreds of data blocks with the same priority class to the same receiver in a very short period of time. Therefore, the TSN value 0-63 will be used repeatedly. The wrap around of finite bit representation of TSNs can cause ambiguity. Without a proper mechanism, the receiver has difficulty to distinguish whether received data blocks belonging to the same cycle or different cycles.
Overall, to combat the stall and wrap around problems, there are two major stall avoidance mechanisms. One is a window based mechanism and another is a timer based one to prevent the stall problem. Of course, several complementary stall avoidance mechanisms exist. Namely, a mechanism uses “In-band acknowledge sequence number (ASN)” and a mechanism uses an in-band MRW bit to indicate the transmission window lower edge.
In brief, referring to FIG. 1—the logical flowchart of the window based prior art stall avoidance mechanism, the prior art mechanism sets up a window of TSNs, where the range of the window is smaller than the whole possible range of TSNs. Usually as indicated in Step 5 the system sets the window size “WINDOW” equal to be half of the transmitter sequence number space as specified. In the case of 6-bit TSN with values between 0-63, the system sets WINDOW=32. In an ideal condition, at any given time the transmitter transmits data blocks having TSN within its window range while the receiver will receive data blocks having TSN within its window range. After the transmitter has transmitted a data block with TSN=SN, the transmitter will not retransmit any data block with TSN≦SN−WINDOW to avoid sequence number ambiguity in the receiver as indicated in Steps 10, 15, 20 and 25. Theoretically, transmitting and receiving data blocks within the smaller window in both the transmitter and the receiver shall reduce the ambiguity caused by TSN wrap around.
Meantime, at the receiver, the receiver initializes its window upper edge equal to WINDOW−1 and its widow lower edge equal to 0 having WINDOW as the total window size (Step 30). When a data block with TSN=SN is received in Steps 35 and 40, if the SN of the data block is within the receiving window (Step 40) and this data block has not previously been received (Step 45), the data block is placed in the reordering buffer at the place indicated by its TSN (Step 55). But if this data block has been previously received it shall be discarded (Step 50). Then as indicated in Step 60, if the SN is outside the receiving window, the received data block shall be placed above the highest received TSN in the reordering buffer, at the position indicated by SN. The receiving window shall be advanced so that the SN forms the upper edge of the receiving window and any data blocks with TSN≦SN−WINDOW shall be removed from the reordering buffer as Step 60 specified. After Step 55 or Step 60, all received data blocks with consecutive TSNs up to the first not received data block are delivered to higher layer (Step 65).
The window based prior art mechanism presents several potential problems. First, a 6-bit TSN prior art system has a fixed window size of 32. The actual window size should be flexibly configured according to UE capabilities. In addition, the latency of received data block will be degraded when uses a larger window size. For example, if negative acknowledgment (NACK) of data block TSN=0 by the Receiver is mistakenly recognized as positive acknowledgment (ACK) by the Transmitter, the receiver need to receive data blocks TSN=1 to 32 successfully before data block TSN=0 can be discarded and other data blocks can be delivered to upper layer if the system uses the fixed window size of WINDOW=32. Second, the criterion of “TSN≦TSN−WINDOW” used at the transmitter (Steps 15, 20 and 25) is ambiguous and how to update the transmission window at the transmitter is not well defined. For example, in a 6-bit TSN system having WINDOW=32, if TSN=SN=63 is transmitted. According to the prior art, TSN≦SN−WINDOW=31 (i.e. TSN=0 to 31) should not be retransmitted. However, whether new data blocks of TSN=0 to 31 can be transmitted is not specified.
Furthermore, in the prior art, the receiving window is advanced only by receiving a data block with the SN outside the receiving window (unconditionally). This rule has the advantage that the transmitter can advance its transmission window without requiring explicit signaling from the transmitter to the receiver. However, this rule will cause potential TSN ambiguity problems. In the following example, Example A, using a 3-bits TSN system with the WINDOW=4 to illustrate, the receiver misses data blocks TSN=0 and 1 and receives data blocks TSN=2 and 3. Then, data blocks with TSN=4 and 5 are received. The upper edge is updated to TSN=5 and the lower edge is updated to TSN=2. Data blocks with TSN=2, 3, 4 and 5 are delivered to upper layers while the receiving window range is between TSN=2 to TSN=5 at this moment. If the receiver receives data block TSN=2 (of the next cycle) and misses data blocks TSN=6, 7, 0, 1. This is possible if NACK for data block TSN=6 is mistaken as ACK by the transmitter so that data block TSN=2 is within the transmission window (TSN=7, 0, 1, and 2 are the transmission window at this moment). According to the prior art, the receiver will take the data block TSN=2 of the next cycle as before data block TSN=5 of the current cycle, recognize the data block of TSN=2 as having been received, and discard it. The system performance is degraded by discarding such a data block prematurely. In fact, the Receiver should update its window's upper edge to TSN=2 in this example (Example A). The prior art has no way to find this correct behavior. In addition, in case of TSN corrupted but undetected by CRC, the prior art is not robust. One more example is used to illustrate the prior art problem, in Example B, using a 6-bit TSN system, data block TSN=0 is missing, data blocks TSN=1, 2 and 3 are received. Data block TSN=4 is corrupted and decoded as TSN=44. Then the missing data block TSN=0 is received after the corrupted data block with TSN=44. By the rules of the prior art, when TSN=44 is received, the receiver will advance the receiving window's upper edge to TSN=44 and the lower edge to TSN=13. Data blocks TSN=1, 2 and 3 are delivered to higher layer. When data block TSN=0 is received, since it is outside the receiving window, the receiver will advance the upper edge of the receiving window to TSN=0. Thus, finally, data block TSN=0 will be delivered to higher layer after the delivered data blocks TSN=1, 2 and 3. An out-of-sequence delivery error occurs.